1. Field of the Invention
This invention relates to recording sheets, for example, a paper based recording sheet, having enhanced print quality. The invention also relates to methods of making and methods of using the recording sheets. While suitable for use in any printing process, the recording sheets are particularly useful in ink jet printing processes.
2. Discussion of the Background
Paper substrates having the so-called, “I-Beam” structure have been recently developed and are reported to have improved bulk stiffness and/or high dimensional stability. See, for example, U.S. Patent Application Publication 2004/0065423, published on Apr. 8, 2004, which discloses a three-layered single-ply I-Beam structure sheet with a cellulosic central layer and top and bottom layers having starch-based size pressed coatings. See also U.S. Patent Application Publication 2008/0035292, published on Feb. 14, 2008, which discloses paper substrates having high dimensional stability with high surface sizing and low internal sizing.
Calcium chloride is currently used in ink jet recording media to enhance inkjet print density and dry time. See, for example, U.S. Patent Application Publication 2007/0087138, published on Apr. 19, 2007, which discloses a recording sheet with improved image dry time which contains water soluble divalent metal salts. Other metal salts have been used in ink jet recording media. U.S. Pat. No. 4,381,185 discloses paper stock which contains polyvalent metal cations. U.S. Pat. No. 4,554,181 discloses an ink jet recording sheet having a recording surface which includes a water soluble polyvalent metal salt. U.S. Pat. No. 6,162,328 discloses a paper sizing for ink jet printing substrate that includes various cationic metal salts. U.S. Pat. No. 6,207,258 discloses a surface treatment composition for an ink jet printing substrate which contains a divalent metal salt. U.S. Pat. No. 6,880,928 discloses an ink jet recording base paper having a coating which includes a polyvalent metal salt.
The present inventors have found that the use of calcium chloride can be problematic. High levels of calcium chloride can create runnability issues in paper machines; calcium chloride undesirably quenches stilbene-based optical brighteners such as often used at the size press; and calcium chloride affects the pH of size press formulations. Starches used at the size press require a narrow pH range to be effective: too high of a pH may result in the yellowing of the starch; too low of a pH may cause the starch to precipitate and/or gel. Calcium chloride can also interact with other chemicals such as those used in the wet end when the paper is broked or recycled.
There is thus a need for a recording sheet in which improved ink jet print density and other benefits are maintained but which avoids the runnability and formulation issues associated with calcium chloride.